Biodegradable polymers have become increasingly important for a variety of biomedical applications including tissue engineering scaffolds. However, relatively few biodegradable, particularly elastomeric, polymers have been developed which are presently in use.
Polymeric medical devices which intentionally degrade and disappear upon completion of their function may mitigate the inevitable, usually negative physiologic responses (eg. fibrous encapsulation) which may limit long-term device success. Thus, an array of degradable polymers have been developed and studied for many uses. However, relatively few of these degradable materials are elastomeric polymers. Rather, the majority of degradable polymers are essentially hard, brittle materials for drug delivery uses. With the increasing interest in tissue engineering degradable materials exhibiting a wide variety of physical properties are necessary to integrate with the various tissues of the body.
Segmented polyurethane elastomers have enjoyed wide use as biomaterials due to their excellent mechanical properties and great chemical versatility. The vast majority of research devoted to the development of biomedical polyurethanes has focused on long-term applications such as vascular grafts and pacemaker lead insulators.
Despite the progress thus far in the development of polyurethanes, relatively little research has been directed at developing intentionally degradable polyurethanes for temporary implantation. Several papers were published in the early 1980's describing polyurethane/polylactide blends as degradable materials for skin substitutes, vascular prostheses and nerve regeneration guides. However, in these cases the polyurethane portion of the blend was non-degradable and served only to provide favorable mechanical properties. Subsequent work by Bruin et al., “Design and Synthesis of Biodegradable Poly(Ester-Urethane) Elastomer Networks Composed of Non-Toxic Building Blocks,” Makromol. Chem., Rapid Commun., 9, 584-594, (1988) involved the synthesis of crosslinked polyurethane networks incorporating lactide or glycolide and .epsilon.-caprolactone joined by a lysine-based diisocyanate. These polymers displayed good elastomeric properties and were found to degrade within 26 weeks in vitro and 12 weeks in vivo (subcutaneous implantation in guinea pigs).
However, a drawback of this approach is that the highly crosslinked polymer may not be processed by standard techniques such as solution casting or melt processing as is the case for typical linear, segmented polyurethanes. Cohn et al developed a series of elastomeric polyester-polyether-polyurethane block copolymers intended for use as surgical articles (EP295,055). However, these polymers are relatively stiff, low tensile strength materials, which may preclude their use as elastomeric biomaterials.
In recent years there has developed increased interest in replacing or augmenting sutures with adhesive bonds. The reasons for this increased interest include: (1) the potential speed with which repair might be accomplished; (2) the ability of a bonding substance to effect complete closure, thus preventing seepage of fluids; and (3) the possibility of forming a bond without excessive deformation of tissue.
Studies in this area, however, have revealed that, in order for surgical adhesives to be accepted by surgeons, they must possess a number of properties. First, they must exhibit high initial tack and an ability to bond rapidly to living tissue. Secondly, the strength of the bond should be sufficiently high to cause tissue failure before bond failure. Thirdly, the adhesive should form a bridge, preferably a permeable flexible bridge. Fourthly, the adhesive bridge and/or its metabolic products should not cause local histotoxic or carcinogenic effects.
Isocyanate-based adhesive/sealant compositions are disclosed, for example, in U.S. Pat. Nos. 6,894,140; 5,173,301; 4,994,542; and 4,740,534, the disclosures of which are incorporated herein in their entirety by this reference.
A number of adhesive systems such as alkyl cyanoacrylates, polyacrylates, maleic anhydride/methyl vinyl ethers, epoxy systems, polyvinyl alcohols, formaldehyde and gluteraldehyde resins and isocyanates have been investigated as possible surgical adhesives. None has gained acceptance because each fails to meet one or more of the criteria noted above. The principal criticism of these adhesives systems has been the slow rate of reaction and potential toxicity problems they pose.